Electric discharge lamp



Dec. 22, 1953 H. H. HOMER ELECTRIC DISCHARGE LAMP Filed Sept. 28, 1949Horace Homer;

INVEN TOR.

Hf: After-neg.

Patented Dec. 22, 1953.

ELECTRIC DISCHARGE LAMP HoraceH. Homer, Arlington, Mass., assignor toSylvania Electric Products Inc., Salem, Mass., a corporation ofMassachusetts Application September 28, 1949, Serial No. 118,252

9 Claims.

1 This invention relates to the prevention of discoloration and increaseof life in electric discharge lamps with oxide-coated cathodes, andparticularly in fluorescent lamps.

I find that the addition of a small amount of hafnium oxide to thecathode coating greatly reduces such discoloration and increases thelife of the lamp.

The figure shows a fluorescent lamp according to the invention, partlyin section.

In that figure, an elongated tubular envelope I of light-transmittingmaterial such as glass, has a stem 2 sealed to each end, with thelead-in wires 3, 4 sealed therethrough and carrying a cathode 5 at eachend, which may be a doublycoiled tungsten wire with an electron-emittinmaterial held between the turns of its minor coil, as shown, forexample, in U. S. Patent 2,258,158, issued October 7, 1941, to Erwin F.Lowry. A

coating 6 of fluorescent material, for example of a calciumhalo-phosphate activated with antimony and manganese, is applied to theinner surface of envelope l in the manner customary in the art. A smalldrop of mercury 9, and a filling of an inert gas, such as argon orkrypton,

at reduced pressure, is sealed into the envelope I.- For example, the!filling may be argon ata pressure of a few millimeters of mercury, apressure of the order of three millimeters being generally preferable.

During manufacture, the exhaust and filling of the lamp is effectedthrough exhaust opening I, in communication with exhaust tube 8, shownsealed off as it would be after the manufacture of the lamp.

A base In is affixed to the ends'of the envelope I, and may be heldthereto by the cement II. The base itself may be of plastic, if desired.The contact pins l2, l3 project. through the base 10, and are connectedto the lead-in wires 3, 4.

A coating of electron emitting material may be applied to the cathode,for example as in the Patent 2,258,158 previously mentioned. Theelectron emitting material is referred to as a coating, for convenience,although it is preferably just 'a quantity of material held between thesmaller turns of the coil 5, as in the patent mentioned.

The coating material may comprise one or more of the alkaline earthoxides, preferably barium, strontium and calcium oxides, together with asmall quantity of hafnium oxide. The alkaline earth oxides may beapplied to the electrode as carbonates and later reduced to the oxidesduring the exhausting of the tube, For example, an

. thereof.

intimate, finely powdered mixture of 58% barium carbonate, 35% strontiumcarbinate, and 7% calcium carbonate will be satisfactory. A small amountof a nitrocellulose lacquer may be added as a vehicle for applying thecoating to the oathode, the vehicle being removed by drying anddecomposition during the exhaust process.

A small amount of finely-powdered hafnium oxide is added to thecarbonates and intimately admixed therewith. The amount of hafnium oxideis preferably in a molal ratio to barium of about 1 to 5.5, although itmay vary in amount between about 0.2 to 5.5 and 2 to 5.5.

After the stem 2 bearing the coated cathode 5 is sealed to the end ofthe envelope I, the lamp is exhausted and filled with'gas in the usualmanner, through exhaust tube 3, prior to the sealing During this processthe cathode is heated, for example by the passage of currenttherethrough, to convert'the carbonates into oxides and carbon dioxide,the latter being removed by the exhaust, leaving the white oxides on thecathode.

I have tried many oxides of metals similar in some respects to hafnium,but the results were unsatisfactory. For example, the tests summarizedin the following table show that the addition of oxidesof beryllium,titanium, cerium and thorium were unsatisfactory, whereas the additionof hafnium oxide gave excellent results.

Table Relative discoloration at various hours life Added oxide 48 52 7272 Failed. 36 42 70 Failed- Do.

0 0 4 10 30. 18 36 72 Failed.-. Failed. 26 40 73 Do. 38 40 42 94 D0.

The table clearly shows the superiority of hafnium oxide over the othersimilar oxides in keeping the discoloration low, and also shows thathafnium oxide gave longer life. For example at 4000 hours, the titaniumand cerium oxide lamps had all failed, and at 6000 hours, the hafniumoxide lamps were the only ones still operating, the beryllium, titanium,cerium and thorium oxide lamps, together with those having no suchadditional oxide, having all failed. Moreover, the discoloration inthese latter lamps was of an altogether different order of magnitudethan that in the hafnium oxide lamps. In fact,

the discoloration in the hafnium oxide lamps at 6000 hours-a time atwhich all the other lamps had failed-was only about equal to the averagediscoloration in the other lamps at 475 hours life.

The discoloration in the lamps with other than hafnium oxides added,while somewhat erratic, were roughly about the -samefionrthemverageg asif no oxideat all had been added.

The lamps marked None in the above table had their cathode coating ofthe barium, strontium and calcium mixture previously described herein,while the other lamps had'adfiiixd with the barium, calcium andstrontium exidessaismall quantity of the particular' oxide' specified}the quantity in each case being that which gave-a molal ratio to thebarium oxide of into-5.63. :The ratio of the additional oxide to thebarium oxide appears to be the important relationship-because thediscoloration appears to be due chiefly to the -barium.

The discoloration is "greatest in lamps'containing-mercuryvaporpand myinvention is particularly effective in such lamps.

lfThe lamps nf-the tests summarizdi in' the table were-40 wattfluorescent lamps ina glass envelope 48 inches long and L-inchesdndiameteriwith' a filling of I argon at a pressure'scf about." 3mill'imete'rsr of mercury; :and mercury vapor. Such lamps =operate at'atemperature of: about- 40LC.,

corresponding 'to a; mercury' vapor; pressure of about: 6' microns. The'filament wire was flol tungsten.

1. A rdischarge lamp cathode'icomprising a coiled tungsten wirei and -anel'eetron emitting m'aterial held-by said .wire and "consistingimainly-of-hafniumoxide and barium oxi'deiinmolalzratio -between 0.2' to 5.5anclzto 5.5.

I: 2;: An electric discharge lamp 1 comprising: a sealed tubularlight-transmitting envelope; lead- :in wires sealed therethrough; a; gasfilling ='at-':low

pressure therein, a tungsten wire coil connected between'said' lead inwires, and an :ele'ctron emitting mixture-of powdered hafniumoand bariumx oxides held :in i saidi coil; .the "molal ratio of hafnium to barium:being between" 0.2" to' 5.5:and 2 =-to 5'.5.

3.x:Anelectric rdischarge lamp *comprisingiswa rs'ealed tubular--lighttransmitting erivelope; leadin wires sealed therethrough, a gas fillingat low pressure therein, a tungsten wire coil connected between saidlead-in wires, and an electron-emitting mixture or powdered hafnium andbarium oxides Held in said c'oil'fthe molal ratioo'f hafnium tdbariumbeing about 1' to 5.5.

electric discharge lamp comprising a sealed tubular light transniitting'enveiope, leadin wires sealed -therethrou'gh, a gas-filling-atlowpressuretherein, a tungstenwire coilconnect'ed' between-said lead-inwires; and an electron-emit- --ting--mixture consisting essentially ofhafnium iioxidegbarium oxide' and at least 'one bthenialka- ="=lineearth oxide held inxasa'id coiL-zthermolal irafio and? to 5;5,&sa'id"hafnium and barium oxides constitiitingcthemain portion-'ofithefmixture.

firnni ele'ctric discharge: ilamp comprising 1a sealed tubularlight-transmitting envelope, leadin wires sealed therethrough, agas-filling at low pressure therein, a tungsten wire coil connectedbetween said lead-in wires, and an electron-emitting mixture consistingessentially of hafnium oxide, barium oxide and at least one otheralkaline earth oxide held in said coil, the molal ratio bfihafniu'm'tobarium-"beinghbout mo: 5.5, said h'afnium -and barium oxidesconstitutingthe main portion of the mixture.

6. An electric discharge lamp comprising a sealedtubularlight-transmitting envelope, leadwires'sealed therethrough, a gasfilling at low 'pressurethereinimercury vapor therein, a tunglfi stn wire coil"'connected between said lead-in -wires,"andanelectron-emittingmixture of powderedflhafni nn andbarium oxides held in said coil, themolal ratio of hafnium to barium being between 0.2 to 5.5 and 2 to 5.5.

7. An electric discharge lamp comprising a sealed tubular:light-transmitting envelope, lead- -in-wiressealed therethrough; agasfilling-at -iow 'pi'essure therein, mercury vapor therein, a-tungstenwire coil connectedbetween saidu lead-in 1 wires; and anelectron-emitting mixture of pow- 'dered hafnium and'barium oxidesheld-in-said :coil'; 'the molal. ratio of 1 hafnium to bariumbeing about1 to 5.5.

.1 8.=' An' electric: discharge lamp comprising a sealed tubularlight-transmitting envelope, -leadiwires sea-led therethrough; a gassiilling: at -:low pressure therein, mercury vapor -therein,--.a'tungsten wire coil: connected between: -said dead-in 'wires; and anelectron-=emitting mixture consistessentiallyof hafnium oxide,- bariumoxide and: at'least one other alkaline earth oxide; held .insaidcoil-,ithe molal ratioof-hafnium to barium being between 0.2-'to' 5.5and: 2 to 5.5;said-barium amihafnium oxide constituting themain portion:ofthe mixture. 7

" '9; An "electric "discharge lamp comprising a sealed tubularlight-transmitting envelope, 7 leadin wires sealedtherethrough, agas-filling atlow ypressure therein, mercury vapor therein; atungstenwire: coil connected between said-,leadein wires and anelectron-emitting mixtureconsisting essentially 'of-hafniumoxide,-barium goxide and; at least one otheralkaline eat-rthoxideheldsin saidcoil; the molal ratio of hafnium -to barium 50 being about 1 to5.5, said bariumand-- hafnium --oxide constituting-:themain *portionoftheLmiX- -=ture.

. i-HORACEsI-IQHOMER.

References Cited in'the iile of' thiapatent

1. A DISCHARGE LAMP CATHODE COMPRISING A COILED TUNGSTEN WIRE AND ANELECTRON-EMITTING MATERIAL HELD BY SAID WIRE AND CONSISTING MAINLY